1. Field of the Invention
This invention is generally directed to methods and devices utilized with container ships for loading and discharging standardized cargo shipping containers and more particularly to an automated multi-directional material handling system which may be used to selectively retrieve and discharge containers while a ship is at sea.
2. Brief Description of the Related Art
The most efficient and economical manner of shipping goods over waterways is the use of standardized cargo containers. The containers are designated in standard sizes which are generally twenty or forty feet in length. The containers are specifically designed so that they may be loaded into the holds and on the decks of ocean going vessels and off loaded from the vessels at a port by use of either on-board or on-shore cranes which place the containers directly onto land transport vehicles including railway cars and trucks. Container ships are specialized vessels which are specifically designed to maximize the storage capacity of international storage and shipping containers. Conventional container ships include one or more hold areas extending from the bow to the stern of the ship with each hold area being divided into a plurality of vertically tiered cells. The cells are defined by vertical steel beams which act as guides for the corners of the containers such that the containers may be stacked one upon another within each cell. Typical cells may retain as many as six or seven stacked containers.
With conventional container ships, the cells are covered by hatch covers or plate which are removed in order to allow a crane to access the uppermost container in each cell. Typically, a container ship will include one or a plurality of either fixed or mobile bridge cranes which have hoists for selectively elevating the containers from the cells and for lowering the containers into the cells. When fixed cranes are provided sufficient cranes must be positioned at various areas spaced along the deck in order to allow access to each cell. The use of overhead bridge cranes permits a single crane to be moved on parallel guide rails. The bridge cranes, are moveable laterally from side to side along the bridge structure such that the cranes are moveable in a horizontal plane.
There are several drawbacks with respect to current container ship load handling or transfer systems. Utilizing conventional systems it is not possible to easily retrieve a specified container which may be located at a bottom of a cell without requiring the removal and temporary placement of other containers along the deck of the ship. During rough sea conditions, such retrieval is not possible. Further, the manner in which the containers are loaded and off-loaded requires the ship to be stabilized and thus either be at a dock or be in an offshore area with quiet seas which permit loads to be elevated safely from the cell structures.
A further drawback of current container ship material handling systems which utilize bridge cranes for elevating and lowering the shipping containers is that only a single bridge crane can operate over a given area of the ship at any one time, thus slowing the rate at which containers can be retrieved or stowed relative to the cells of the container ship.
In view of the foregoing, it would be beneficial to have a material retrieval and handling system which could be used on container ships whereby selected containers may be rapidly and easily retrieved regardless of their position within a cell of the ship and regardless of sea conditions. Further, it would be beneficial to have such a system wherein multiple container handling units could operate over a common grid structure so that a plurality of containers may be simultaneously moved with respect to the cells of the various holds of the container ship.